In recent years, as a technology for reducing fuel consumption of vehicles, vehicles have been developed, which recover kinetic energy at a time of a decrease in speed of vehicles as electric power by power regeneration. In this technology, motive power is transmitted by a drive belt looped around an internal-combustion engine and a power generator. However, when power-generation torque is increased so as to obtain a larger power-regeneration amount, the drive belt may slip due to large force applied thereto.
When feedback control is performed with use of generated power as a command value, in the case where a belt slip occurs, the generated power decreases due to a decrease in revolution speed of the power generator, and a field current increases when feedback control is performed so as to maintain the generated power at the command value. Further, in the case where the grip of the drive belt is recovered, the original revolution speed of the power generator is recovered while the field current is output as a result of the feedback control performed so as to set the generated power at the command value with respect to the decreased the revolution speed of the power generator. Therefore, there is a risk in that the power-generation amount of the power generator rapidly increases to damage equipment. In Patent Literature 1, the above-mentioned point is referred to as a problem, and an apparatus disclosed in Patent Literature 1 solves the above-mentioned problem by limiting, when the belt slip occurs, a field current to a field current value calculated from the revolution speed of an engine, that is, a revolution speed of the power generator at a time of recovery of grip so that generated power becomes a command value, thereby preventing a rapid increase in power-generation amount of the power generator.